The implementation of a system of large-scale actuator arrays and sensors for the contour and control of surfaces for space or other applications, presents significant challenges. Among these challenges are the following:                Structure size and mass density—For mobile or space-borne applications, control surfaces should be weight efficient and capable of repeatable deployment (and possibly re-stowage) from a small volume. For reflector antenna applications, extremely large apertures offer unique performance characteristics. These expansive, gossamer structures will most likely require actively-controlled actuator arrays to stabilize the surface. The power and control network must be similarly mass efficient and not interfere with the deployment or stability of the structure.        Power transfer efficiency and isolation—An array of actuators and sensors distributed over a very large surface should be powered and controlled efficiently with minimal interference with the system's mission, nearby systems and subsystems collocated on the associated platform.        Surface precision and actuator density—As a structure decreases in stiffness which is typical of large deployable systems with low mass density, the more significant the higher structural modes become. For antenna reflector and optical surfaces, the higher the mission frequency the more precise the surface must be maintained. In either case, increasing the number and/or complexity of the actuators on the surface will likely be required. The higher number and greater functionality of the actuator array, complicates the corresponding power and control network.        
The present invention, which preferably takes advantage of rectennae, may be deployed as a power and control network for a large active structure, thereby addressing each of the above issues by employing highly localized RF surface waves propagating across a meta-surface to transmit signals for power and command. Using these surface waves, in effect, creates a 2-D wireless network that is well-suited for active surfaces. This approach has much of the configuration flexibility of a standard broadcast-type wireless system but with significantly greater power efficiency and signal isolation.
The present invention permits, but does not require communication and power transfer between a central control module and an array of distributed actuators; being especially useful when the actuators are distributed over a large-scale adaptive surface. It provides a way of increasing the signal power in the region where the sensors are located, and minimizing the amount of radiation that is lost to free space.
Surface wave Power is collected at an actuator with a surface wave rectenna. A rectenna is a known device:                (i) a rectenna (RECTifying antENNA), is a special type of antenna that is used to directly convert microwave energy into DC electricity. Its elements are usually arranged in a mesh pattern, giving it a distinct appearance from most antennae.        (ii) a simple rectenna can be constructed from a Schottky diode placed between antenna dipoles. The diode rectifies the current induced in the antenna by the microwaves. Schottky diodes are used because they have the lowest voltage drop and therefore waste the minimum power.        (iii) rectennae are highly efficient at converting microwave energy to electricity. In laboratory environments, efficiencies above 90% have been observed with regularity.        
See U.S. patent application Ser. No. 10/944,032 filed 17 Sep. 2004 for information about rectenna designs using a Frequency Selective Surface (FSS).
Its advantages over other systems and methods are:                that it can be an integral part of the active surface, thus minimizing complexity.        the surface waves used to transfer power and data throughout the surface are tightly bound to the surface without field leakage into surrounding space, unlike wireless methods where a large portion of the signal radiates into space, thereby wasting power, and reducing efficiency. This is especially critical when power is being transmitted wirelessly.        that it does not require high-precision pointing and tracking hardware such as are necessary with optical methods that are often proposed for wirelessly transmitting power via highly directive lasers.        that it does not require a network of wires running over the surface such as is used in methods where direct connections are made between the control point and the actuators.        that the meta-surface is potentially mass produced and therefore inexpensive.        it provides robustness against single-point breaks, which would be a problem for a network of wires, so it is robust against localized damage.        the single conductor, surface wave method for distributing power provides protection against shorts, which would otherwise be a problem for a parallel system like this.        
The invention can be applied to actuator arrays on large-scale adaptive antennas used to provide reconfigurable beam-gain profiles for satellite broadcasts, for example. Large scale antennas can be 1 meter and up in size. A flexible reflector with actuators that can deform the reflecting surface in response to the beam quality, based on the reported signal quality at various locations on Earth, should prove to be quite useful. Another potential application is with mobile ground-based RF antennas and optical structures that must packaged and transported within a small volume on a space vehicle, then deployed and controlled when in operation in space.
Another application is the control and monitoring of an array of passive sensors that, for example, monitor temperature, pressure, material strain or any other physical parameter.
Additionally, the disclosed surface-wave medium can be integrated within structural components (walls, etc.) to create a multifunctional structure that provides communication and power of distributed sensors and actuators.
The surface waves enable addressing an array of sensors without the complexity, cost and low reliability of wiring harnesses and power and data busses. The sensors can be embedded within the structure or retrofitted to it after fabrication and reconfigured as necessary. This multifunctional structure allows advanced control and monitoring functions without the detriments of the attendant complexity, weight and failure modes of wired networking and power.
The surface-wave control, communication and power functionality advantageously provides the benefits of (1) integrating the power and control into a single ‘bus’, (2) implicit built-in redundancy and tolerance to damage, (3) safety-critical capability where the sensors are placed in harsh environments not amenable to conventional wiring harnesses thus improving reliability and increasing structural and system health monitoring functionality, (4) reducing vehicle weight and complexity by reducing the number of wiring harnesses by 10 to 15%, (5) facilitating common sensor deployment that leads to reduced maintenance cost and improved dispatch reliability of several percentage points, and (6) simplifying the design of vehicle power and electrical distribution systems.
These attributes would uniquely enable multi-node sensing systems such as those required for the integrated health management of existing and new space and air vehicles.
One important feature of this invention is to simultaneously provide power and control signals to a network of sensors and/or actuators distributed over a large surface, as in a large aperture reflector. This can be accomplished without the use of power and control wires which complicate the architecture, add mass, and may interfere with the deployment and function of the structure (if they bind or tangle).
Other wireless systems have been proposed, including:                1) A free-space RF broadcast system capable of providing both control signals and power when rectified. However, this technical can be inefficient if the transmit beam angle is large and the broadcast frequency is high. In this case, the power available to the actuator is very low due to the small size of the receive antenna compared to the beam footprint at the surface. Much of the transmitted power is therefore lost. Another potential problem is that the radiated energy may interfere with sensitive systems on or near the platform or inadvertently reflected off the surface and interfere with the platform's mission. Chadwick and Hughes (WO 03/009501) propose a more efficient wireless system within an evanescent field defined by an enclosed space. In many cases though, the surface does not form an enclosed space, and much of the benefit is lost.        2) An optical system wherein power and commands transmitted by a laser and received by a photovoltaic device at each actuator. Miller (U.S. Pat. No. 5,709,245), Hagans et al (U.S. Pat. No. 6,055,079) and others show methods and devices for transmitting power and simplified command information optically to a photovoltaic device. On one hand, if the beam is focused sufficiently to provide enough power for reasonable operation of the device, the collimated beam may have to be mechanically steered and possibly capable of acquiring and tracking photoreceptors at multiple locations over a large surface. This process often requires complex and precise gimballing mechanisms and tracking systems. On the other hand, if the beam is defocused to cover the range of motion for a significantly large surface, the output power of the laser must be increased to provide a comparable power density at the photovoltaic device.        3) Using an electron gun to remotely charge piezoelectric flexural actuators located on the control surface. See Martin et al. (U.S. Pat. No. 6,297,579). The electron beam can be directed without using gimbals, and control and power electronics at each actuator are not required. However, actuator options with this method are limited and possibly prohibitive. Also, as with all focused beam concepts, more complex acquisition and tracking may be required as structural range-of-motion and/or actuator density increases.        